(1) Field of the Invention
The present invention generally relates to a duplex communication path switching system, and more particularly to a duplex communication path switching system applicable to a ring network which can effectively switch a duplex communication path for data transmitted at any speed.
(2) Description of the Related Art
In a ring network, a communication path is duplicated in order to improve reliability of communication. That is, a clockwise transmission line and a counterclockwise transmission line link respective nodes together, at which nodes terminals are accommodated. A direction in which data is transmitted through the clockwise transmission line is opposite to a direction in which data is transmitted through the counterclockwise transmission line. At each node, signals from terminals are supplied to both the clockwise transmission line and the counterclockwise transmission line so as to be transmitted through both the transmission lines in opposite directions. At a node accommodating destination terminals, normal signals are selected from among signals received from the clockwise transmission line and the counterclockwise transmission line. The above communication system is referred to as a transmission-hybrid/receive-selecting system. At a receiver-side node, to select the normal signals, the communication path is switched from the clockwise transmission line to the counterclockwise transmission line or from the counterclockwise transmission line to the clockwise transmission line. A system for switching the communication path between the clockwise transmission line and the counterclockwise transmission line is referred to as a duplex communication path switching system.
FIG. 1 shows an example of the ring network. Referring to FIG. 1, the ring network has a clockwise transmission line LR and a counterclockwise transmission line LL which link nodes A, B, C and D. For example, at the node A, a signal received from a terminal 100 is supplied to both the clockwise transmission line LR and the counterclockwise transmission line LL via a hybrid connector 101. The communication path is switched by use of a selective connector 102 between the clockwise transmission line and the counterclockwise transmission line so that a normal signal is selected, as a signal to be received by the terminal 100, from among signals transmitted through the transmission lines LL and LR. At the node B, in the same manner as at the node A, a signal received from a terminal 200 is supplied to the clockwise transmission line LR and the counterclockwise transmission line LL via a hybrid connector 201, and the switching operation between the clockwise transmission line LR and the counterclockwise transmission line LL is performed by use of a selective connector 202 so that a normal signal is selected as a signal to be received by the terminal 200 from among signals transmitted through both the transmission lines LR and LL. The other nodes C and D have the same structure as the nodes A and B described above.
In order to determine which received signal is to be selected as the normal signal from the clockwise transmission line or from the counterclockwise transmission line, a frame signal added to the received signal or error checking information is checked. This checking operation is performed for every unit of transmission signal (this unit is referred to as a switched-signal unit).
In a case of the conventional PDH (Presynchronous Digital Hierarchy), a Handling Group (HG) of 64 kbpsxc3x976, a unit of 1.5 Mbps or a unit of 6.3 Mbps is used as the switched-signal unit. In a case of the SDH (Synchronous Digital Hierarchy), a unit of multiplexed signals, such as VC1 or VC22, is used as the switched-signal unit.
At each node, signals from a plurality of terminals are multiplexed. To multiplex the signals, circuits (e.g., multiplexers) for multiplexing the signals are hierarchically coupled to each other. Circuits in the lowest hierarchy are coupled to the respective terminals and output signals each corresponding to the switched-signal unit. A circuit in the highest hierarchy is coupled to the communication path (both the clockwise transmission line and the counterclockwise transmission line). Each of the circuits is duplicated to improve the reliability. If an error is detected in a multiplex signal in a circuit, an alert signal is supplied to circuits in lower side hierarchies so that the circuits in the lower side hierarchies are respectively switched to other circuits. If an error is detected in a signal in a circuit in the lowest hierarchy, only the circuit in which the error has occurred is switched to another circuit.
Meanwhile, when the communication path is switched between the clockwise transmission line and the counterclockwise transmission line in a receiving-side node, an accurate communication signal must be obtained in the receiving-side node. Thus, in the conventional duplex communication path switching system, a transmission speed in the communication path between nodes is limited to the speed (bps:bits per second) of the switched-signal unit of the signal.
There are two kinds of switching logic, 0/1-system switching logic and N(normal)/E(emergency)-system switching logic.
In the 0/1-system switching logic, one of a 0-system and a 1-system is selected as a normal system. For example, the 0-system is selected as the normal system. When the 0-system malfunctions, the 0-system is switched to the 1-system. After this, the 1-system is continuously selected as the normal system although the 0-system is recovered. In the N/E-switching logic, an N-system is usually selected as the normal system. When the N-system malfunctions, the N-system is switched to the E-system. After this, if the N-system is recovered, the E-system is switched back to the N-system.
The communication path may be switched between the clockwise transmission line and the counterclockwise transmission line in accordance with the 0/1-system switching logic. In this case, the clockwise transmission line and the counterclockwise transmission line are used as the 0-system and the 1-system flexibly. As a result, if a signal in a line with a transmission speed corresponding to a plurality of switched-signal units is transmitted through the communication path, switched-signal units from the 0-system and the 1-system may be arranged at random in a received signal.
The communication path may be switched between the clockwise transmission line and the counterclockwise transmission line in accordance with the N/E-system switching logic. In this case, the clockwise transmission line and the counterclockwise transmission line are used as the N-system and the E-system fixedly. As a result, when the N-system malfunctions and is then recovered, switched-signal units from the N-system and the E-system are mixed in a received signal.
A delay time in the clockwise transmission line between nodes differs from a delay time in the counterclockwise transmission line between the nodes. Therefore, a received signal in which switched-signal units from the clockwise transmission line and the counterclockwise transmission line are mixed may be incorrect. That is, communication between terminals accommodated at different nodes coupled to the clockwise transmission line and the counterclockwise transmission line may be not accurately performed.
For example, in a case where the HG (the Handling Group in the PDH) is used as the switched-signal unit, a communication path having a speed greater than 384 (64xc3x976) Kbps corresponding to the HG cannot be formed in the ring network. However, in recent years, a data communication speed is improved, and it is desired that a communication path in which switched-signal units corresponding to various speeds (from a low speed to a high speed) are formed.
Accordingly, a general object of the present invention is to provide a novel and useful duplex communication path switching system in which the disadvantages of the aforementioned prior art are eliminated.
A specific object of the present invention is to provide a duplex communication path switching system capable of switching a duplex communication path between a clockwise transmission direction and a counterclockwise direction so that units are accurately selected from the clockwise transmission line and a counterclockwise transmission line
The above objects of the present invention are achieved by a duplex communication path switching system provided at each of nodes connected by a communication path in a ring network, the communication path having a first transmission line through which a signal is transmitted in a first direction and a second transmission line through which a signal is transmitted in a second direction opposite to the first direction, a signal formed of a plurality of units being supplied to the first transmission line and the second transmission line in parallel to transmit the signal from one node to another through the communication path, the duplex communication path switching system comprising: checking means for checking whether or not each of the plurality of units of the signal received from each of the first and second transmission lines is normal; and switching means for switching the communication path between the first transmission line and the second transmission line based on checking results obtained by the checking means so that units which are normal are selected from the signal, wherein each of the plurality of units of the signal has a constant speed and is provided with frame information defined for the signal transmitted through the communication path, the frame information being used to divide the signal into the plurality of units.
According to the present invention, the communication path is switched using the frame information provided in each of the units of the signal. Thus, the communication path can be switched between the first and second transmission lines so that units are accurately selected from the first and second transmission lines.
Another object of the present invention is to provide a duplex communication path switching system capable of switching the duplex communication path under a condition in which a transmission speed of the duplex communication path between nodes in a ring network is not limited to a transmission speed of each switched-signal unit which is a signal unit checked as to whether the duplex communication path is to be switched.
The above objects of the present invention are also achieved by a duplex communication path switching system in which each of a plurality of units of the signal is provided with specific frame information which is specific to the each of the plurality of units, and the specific frame information is used to divide the signal into the plurality of units instead of the above frame information.
According to the present invention, the communication path is switched using the specific frame information provided in each of the units of the signal. Thus, even if the units of the signal have various speeds, the duplex communication path can be switched so that the units are accurately selected from the first and second transmission lines. That is, the communication path can be switched under a condition in which a transmission speed of the duplex communication path between nodes in a ring network is not limited to a transmission speed of each unit which is checked whether the duplex communication path is to be switched.